As is well known, polyacetal resins have been used extensively in various fields as an engineering plastics material due to the excellent physical properties (such as mechanical and electrical properties) and chemical properties (such as chemical resistance and heat-resistance properties) that such resins possess. As a result, polyacetal resins have been used as a material to form component parts of electric and electronic apparatus, automobiles, precision instruments, building materials (e.g., pipes) and the like.
Notwithstanding the excellent inherent properties exhibited by polyacetal resins generally, there is still a need for continual improvements to be made in this regard, especially as new and/or more specialized end-use applications for polyacetal resins are identified. For example, attempts to improve the weather resistance and/or electrical conductivity properties of polyacetal resins have included incorporating (e.g., via melt-blending) carbon black within a polyacetal base resin. Incorporation of carbon black within a polyacetal base resin, however, has several disadvantages.
In this connection, the presence of carbon black in polyacetal resin results in a decrease in the heat stability of the composition. For this reason, when relatively large amounts of carbon black are incorporated into polyacetal resin (e.g., to impart improved weather resistance and/or electrical conductivity properties) during production of a carbon black-laden master batch, the polyacetal resin can be degraded to an extent that significant free formaldehyde gas is released into the ambient environment. The evolution of formaldehyde gas in the workplace is, of course, to be avoided. Furthermore, incorporation of meaningful amounts of carbon black in polyacetal resin will typically result in tarry substances being deposited on mold cavity walls when the carbon black-laden polyacetal resin is subjected to molding operations. Thus, over prolonged time periods, these mold deposits will build up to an extent that affects the quality of the resulting molded articles requiring costly equipment down time to allow for cleaning
There is also an increased demand for plastics to be recycled (for example, scrap materials, such as sprues, runners and the like, which result during molding of useful articles/components). This increased demand for "recyclable" plastics has occurred not only as a result of a desire to economize resources and reduce ecological waste, but also to reduce raw material costs for manufacturers of plastics articles. However, recycling plastics materials, especially polyacetal, further deteriorates the heat stability and other mechanical properties of the resin.
It has also been proposed to incorporate various heat-stabilizers, such as phenolic, amine or urea stabilizers, into polyacetal resin as a means to increase its heat-stability properties, particularly during molding. Although incorporation of such stabilizers does impart some improvements to the heat-stability properties of polyacetal resin, especially polyacetal resins which additionally include carbon black, further improvements are still needed. It is towards fulfilling such needs that the present invention is directed.
Broadly, the present invention is directed to recyclable carbon black-laden polyacetal resin compositions which exhibit improved melt-stability. More particularly, the present invention is directed to polyacetal molding compositions which include a normally solid polyacetal base resin, and between about 0.1 to 30 parts by weight of carbon black homogeneously dispersed throughout the polyacetal base resin in an ethylenic carrier resin. Preferably, the ethylenic carrier resin is present in an amount between 0.3 to 8 times the amount by weight of the carbon black.
The compositions of this invention are also especially characterized by the presence of a co-stabilization package which includes (1) a first stabilizer which is selected from nitrogen-containing compounds, fatty acid ester compounds, and metal-containing compounds which include a hydroxide, an inorganic acid salt or a carboxylic acid salt of an alkali metal or an alkaline earth metal, and (2) a second stabilizer which is a hindered phenolic compound.
Further aspects and advantages of this invention will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments.